Polyethylene has desirable properties that have helped to make it the highest volume polymer manufactured. Polyethylene can be made in different processes in order to give different properties. Known families of polyethylene include high density polyethylene (HDPE), linear low density polyethylene (LLDPE), and low density polyethylene made using high pressure reactors (LDPE). Within these broad classes many variations exist resulting from different types of polyolefin process technologies (for example, solution, slurry or gas phase) or from the use of different catalysts (for example, Ziegler-Natta or constrained geometry catalysts). The desired application requires a careful balance of rheological properties which will lead a person of skill in the art to select one type of polyethylene over another. In many applications, such as blow-molding and blown film applications, melt strength of the polyethylene is a key parameter, frequently measured as elongational viscosity of the polymer.
The melt strength is a practical measurement that can predict material performance when submitted to elongational deformations. In melt processing good melt strength is important to maintain stability during processes such as coating, blown film production, fiber spinning and foamed parts.
Melt strength is related to several processing parameters such as bubble stability and therefore thickness variation during blown film production; parison formation during blow molding; sagging during profile extrusion; cell formation during foaming; more stable thickness distribution during sheet/film thermoforming.
This property can be enhanced by using resins with higher molecular weight, but such resins will generally require more robust equipment and more energy use because they tend to generate higher extrusion pressure during the extrusion process. Therefore, properties must be balanced to provide an acceptable combination of physical properties and processability.
In thick film applications, such as for use in silage applications, blends of LDPE and LLDPE are typically used in order to obtain a balance of processability (extruder amps and pressure) and film mechanical properties. In this blend the LDPE component is the processability component whereas the LLDPE is the mechanical end component. Therefore, the ability to decrease the LDPE portion of the blend should increase the mechanical properties of the blend. Through this invention, the ability to increase the melt strength of the LLDPE component allows the use of a higher percentage of LLDPE in the blend, thus increasing the mechanical properties without sacrificing processability.
Accordingly, one aspect of the invention is a film particularly well suited for thick film applications. For purposes of the present invention a “thick film” is one having an average thickness of at least 100 microns, and for many applications one having an average thickness of greater than 200 microns. The films of the present invention comprise a polyethylene which has been reacted with an alkoxy amine derivative through regular extrusion processing.
Accordingly, one aspect of the invention is a film having a thickness greater than 200 microns comprising a polyethylene polymer made by the process of first selecting a target polyethylene resin having a density, as determined according to ASTM D792, in the range of from 0.90 g/cm3 to 0.955 g/cm3, and a melt index, as determined according to ASTM D1238 (2.16 kg, 190° C.), in the range of from 0.01 g/10 min to 10 g/10 min. Then the target polyethylene is reacted with an alkoxy amine derivative in an amount less than 900 parts derivative per million parts by weight of total polyethylene resin under conditions sufficient to increase the melt strength of the target polyethylene resin. This modified target resin is then combined with an amount of low density polyethylene prepared in a high pressure process, and the blended resin is then used to make a film.
The modified target resins for use in the present invention increase the elongational viscosity at low (0.1 s−1) shear rates while maintaining the viscosity at higher shear rates (>100 s−1) such that the ease of processing of the material is maintained at typical extrusion conditions. One aspect of the invention is that the extruder pressure does not increase more than 10% of the comparative resin upon processing the inventive resin at the same operating conditions.